1. Field of the Invention
The present invention relates to a method and applicator for direct or indirect application of a liquid or pasty coating medium onto a traveling material web, notably of paper or cardboard.
2. Description of the Related Art
Applicators are used in so-called coating systems for providing a traveling material web, formed for example of paper, cardboard or a textile material, on one or both sides with one or several layers of the coating medium, for example color, starch, impregnating fluid or the like.
In the so-called direct application, the liquid or pasty coating medium is applied by an applicator system directly onto the surface of the traveling fiber material web. The web is carried during application on a rotating countersurface, for example an endless belt or a backing roll. In the indirect application of the medium, the liquid or pasty coating medium is first applied onto a substrate, for example the surface of a backing roll configured as an applicator roll. The coating medium is then transferred from the applicator roll to the material web, in a nip through which the material web passes.
Known from German Patent Document No. DE 43 36 365 A1 is an applicator for direct application of a liquid or pasty coating medium onto a traveling material web, notably of paper or cardboard. The applicator includes a metering slot configured as an open-jet nozzle and from which issues the coating medium in an open jet extending through the ambient atmosphere. The applicator also includes a countersurface disposed opposite the open-jet nozzle, in the form of the material web supported by a backing roll and acted upon by the open jet. To avoid quality impairments occurring notably at high speeds of the material web due to air entrained in the coating layer, the open jet is directed, according to DE 43 36 365 A1, in a direction opposite to the direction of travel of the material web. Upon impinging on the material web, the open jet splits into a backflow, proceeding in a direction opposite to the direction of travel of the material web, and a mixed flow of coating substance proceeding in the direction of travel of the material web. The portion of backflow achievable of the overall flow of the open jet amounts to about 30 to 60 percent. The applied coating medium is subsequently profiled by a doctor element following the open-jet nozzle. Owing to the above measures, a good penetration of the coating medium into the material web is accomplished pursuant to DE 43 36 365 A1. Further, an entrainment of air in the coating medium, and thus the occurrence of undesirable air bubbles, is avoided.
In conventional applicators of the type described above it has been found that the width of the metering slot forming the open-jet nozzle cannot be configured selectively small without entailing an untidy or disuniform open jet or clogging of the nozzle. This leads to a poor coating result and high maintenance expense. This has been found to be true particularly for the production of low coating weights, that is, small coating quantities or a thin coating. Therefore, with conventional applicators, a larger amount of liquid or pasty coating medium, larger than would be necessary for the desired coating with a low coating weight, must first be applied at surplus to produce a neat coating. The coating medium must thereafter be scraped down again. This, in turn, involves a higher energy demand and a larger size of the applicator and, moreover, requires more expensive accessory systems.
The present invention provides an improved applicator such that the disadvantages associated with the prior art are avoided to the greatest possible extent. Also, the production of a high quality coating with a low coating weight can be easily realized.